Carbon monoxide is a toxic gas formed by incomplete burning of organic materials. Carbon monoxide combines with blood hemoglobin to form carboxyhemoglobin which is ineffective at transporting oxygen to body cells. Inhalation of air containing 1-2% (10,000 to 20,000 ppm) CO by volume will cause death within several minutes. CO concentrations higher than 1200 ppm are considered immediately dangerous to life and health by the U.S. National Institute of Occupational Safety and Health (NIOSH).
CO is responsible for many of the fatalities in fires. It is also encountered in mining operations in which explosives are used in confined spaces. CO is also present in the exhausts of gasoline or diesel powered internal combustion engines. Poorly operating engines, machinery, heating equipment, ventilation equipment, air conditioning equipment, and other equipment may also output CO, contaminating the air in buildings and vehicles. Consequently, there is a strong need for protection against CO in these and other environments in which persons could encounter the gas.
Firefighters and other emergency response personnel have been equipped with self-contained respirators using compressed air or oxygen in cylinders to provide protection against CO. These devices tend to be heavy, bulky, expensive and require special training for effective use. It is not feasible to equip everyone in an area with such devices.
A fire or other sudden unexpected release of carbon monoxide in a building, public place, vehicle, or the like may require that individuals quickly escape from an area containing dangerous concentrations of the gas. In these situations, an easy-to-use, lightweight respirator or mask equipped with media capable of protecting against carbon monoxide would be desirable.
Protection against CO is also desirable in the cabin environment of a car, truck, rail-borne vehicle, marine vessel, or other mode of transport. In many heavily congested traffic areas and in tunnels, elevated levels of CO can develop from the accumulation of exhaust emissions. Typically, the CO levels encountered are usually less than 200 to 300 ppm, but even these CO levels can cause headaches, dizziness and nausea to drivers and passengers. In these applications, large volumes of gas and high flow rates can be encountered. Thus, the residence time of the cabin air on the catalyst is short, being less than 0.05 seconds and even less than 0.03 seconds. It is therefore desirable to have a catalyst that can also remove CO under these conditions.
However, the low boiling point and high critical temperature of CO make its removal by physical adsorption very difficult when the CO is present at room temperature. Conventional gas mask canisters and filters based on activated carbon adsorbents have been relatively useless as a practical matter against high concentrations of carbon monoxide.
Catalytic oxidation to carbon dioxide is one feasible method for removing carbon monoxide from air at the high concentrations and flow rates required for individual respiratory protection. However, most CO oxidation catalysts are only active at temperatures of 150° C. or higher. This is true even though oxidation to CO2 is thermodynamically favored. Very few CO oxidation catalysts are active at room temperature or below. A catalyst useful for respiratory protection against CO desirably functions at low temperatures.
Two types of catalysts that are known for low temperature CO oxidation include transition metal oxides (mostly mixed oxides of Cu, Mn, and/or Co) and supported noble metal catalysts. One widely used transition metal oxide for low temperature CO oxidation is hopcalite. Hopcalite is a mixed oxide of manganese and copper developed during World War I by the U.S. Bureau of Mines and the Chemical Warfare Service of the U.S. Army [Lamb, Bray, and Frazer, J. Ind. Eng. Chem., 12, 213 (1920)]. Hopcalite is a very active catalyst for CO oxidation even at temperatures as low as −20° C. The major disadvantage of hopcalite is that its capability for CO oxidation is quickly destroyed by water vapor in the air. This means that a respirator filter with a hopcalite catalyst must include a drier bed on the inlet side of the filter. The useful life of the respirator filter is determined by the capacity and efficiency of the drier bed. Even a filter designed for short term use (˜30 minutes) at high breathing rates will require a desiccant bed of larger volume than the catalyst bed itself. Hopcalite is commercially available from Cams Chemical Company, 315 Fifth Street, Peru, Ill. 61354 USA under the designation Carulite 300.
Catalytic oxidation of CO over supported platinum group metals (most often Pt, Pd, Rh, Ru, and Ir) has been known for many years. However, most of these catalysts are only active at temperatures around 150° C.
In recent years, supported platinum group metal catalysts have been developed that function at lower temperatures. In addition to a platinum group metal, these catalysts may also contain so-called “reducible metal oxides” such as SnOx, CeOx, and FeOx. It is thought that the reducible oxides provide sites that dissociatively adsorb O2, thereby promoting low temperature CO oxidation. U.S. Pat. No. 4,536,375 and Published UK Patent Application GB 2,141,349 discuss these catalysts and their use in respiratory protection devices. A low temperature CO oxidation catalyst of this type is commercially available from Molecular Products Ltd, Mill End, Thaxted, Essex CM6 2LT, United Kingdom under the designation Sofnocat® 423. It contains platinum, palladium, and SnO2.
These platinum-based catalysts are much more tolerant of water vapor than is hopcalite. However, operation at high relative humidity (RH) with low CO inlet concentrations results in capillary condensation of water vapor in the micropores of the catalyst support (usually alumina or silicagel). This causes slow loss of activity as access to active sites is blocked by condensed water. A significant disadvantage of these catalysts is the high loading of expensive platinum group metal necessary to meet the requirements for respiratory protection against CO.
It has been observed that nanoislands of very finely divided gold on reducible oxide supports are very active for CO oxidation at low temperature. At ambient to sub-ambient temperatures, the best gold catalysts are considerably more active for CO oxidation than the most active promoted platinum group metal catalyst known. Gold is also considerably cheaper than platinum. Catalytically active gold, though, is quite different from the platinum group metal catalysts discussed above. The standard techniques used in the preparation of supported platinum group metal catalysts give inactive CO oxidation catalysts when applied to gold. Different techniques, therefore, have been developed for deposition of finely divided gold on various supports. Even so, highly active gold catalysts have been difficult to prepare reproducibly. Scaleup from small lab preparations to larger batches has also proved difficult.
These technical challenges have greatly hindered the industrial application of gold catalysts. This is unfortunate since the very high activities of gold catalysts for CO oxidation at ambient and sub-ambient temperatures and their tolerance for high water vapor concentrations make them otherwise strong candidates for use in respiratory protection filters and in other applications in which oxidation of CO would be desired.
Because ultra-fine particles of gold generally are very mobile and possess large surface energies, ultra-fine particles of gold tend to coagulate easily. This tendency to coagulate makes ultrafine gold hard to handle. Coagulation also is undesirable inasmuch as the catalytic activity of gold tends to fall off as its particle size increases. This problem is relatively unique to gold and is much less of an issue with other noble metals such as platinum (Pt) and palladium (Pd). Thus, it is desired to develop methods to deposit and immobilize ultra-fine gold particles on a carrier in a uniformly dispersed state.
Known methods to deposit catalytically active gold on various supports recently have been summarized by Bond and Thompson (G. C. Bond and David T. Thompson, Gold Bulletin, 2000, 33(2) 41) as including (i) coprecipitation, in which the support and gold precursors are brought out of solution, perhaps as hydroxides, by adding a base such as sodium carbonate; (ii) deposition-precipitation, in which the gold precursor is precipitated onto a suspension of the pre-formed support by raising the pH, and (iii) Iwasawa's method in which a gold-phosphine complex (e.g., [Au(PPh3)]NO3) is made to react with a freshly precipitated support precursor. Other procedures such as the use of colloids, grafting and vapor deposition, have met with varying degrees of success.
These methods, however, suffer from difficulties aptly described by Wolf and Schuth, Applied Catalysis A: General, 2002, 226 (1-2) 1-13 (hereinafter the Wolf et al. article). The Wolf et al. article states that “[a]lthough rarely expressed in publications, it also is well known that the reproducibility of highly active gold catalysts is typically very low.” The reasons cited for this reproducibility problem with these methods include the difficulty in controlling gold particle size, the poisoning of the catalyst by ions such as Cl, the inability of these methods to control nano-sized gold particle deposition, the loss of active gold in the pores of the substrate, the necessity in some cases of thermal treatments to activate the catalysts, inactivation of certain catalytic sites by thermal treatment, the lack of control of gold oxidation state, and the inhomogeneous nature of the hydrolysis of gold solutions by the addition of a base.
In short, gold offers great potential as a catalyst, but the difficulties involved with handling catalytically active gold have severely restricted the development of commercially feasible, gold-based, catalytic systems.
German Patent Publication DE 10030637 A1 describes using PVD techniques to deposit gold onto support media. The support media described in this document, though, are merely ceramic titanates made under conditions in which the media would lack nanoporosity. Thus, this document fails to indicate the importance of using nanoporous media to support catalytically active gold deposited using PVD techniques. International PCT Patent Publications WO 99/47726 and WO 97/43042 provide lists of support media, catalytically active metals, and/or methods for providing the catalytically active metals onto the support media. These two documents, however, also fail to appreciate the benefits of using nanoporous media as a support for catalytically active gold deposited via PVD. Indeed, WO 99/47726 lists many preferred supports that lack nanoporosity.
Relatively recently, very effective, heterogeneous catalyst systems and related methodologies using catalytically active gold have been described in assignee's co-pending United States patent application having U.S. Ser. No. 10/948,012, titled CATALYSTS, ACTIVATING AGENTS, SUPPORT MEDIA, AND RELATED METHODOLOGIES USEFUL FOR MAKING CATALYST SYSTEMS ESPECIALLY WHEN THE CATALYST IS DEPOSITED ONTO THE SUPPORT MEDIA USING PHYSICAL VAPOR DEPOSITION in the names of Larry Brey et al., and filed Sep. 23, 2004, the entirety of which is incorporated herein by reference (hereinafter referred to as Assignee's Co-pending Application). In particular, Assignee's Co-pending Application describes providing catalytically active gold on a composite support derived from relatively fine titania particles (referred to as guest material) that at least partially coat the surfaces of relatively large alumina particles (referred to as host material). These composite systems provide excellent catalytic performance with respect to CO oxidation. However, improvements are still desired. Notably, it would be desirable to provide gold-based catalyst systems that demonstrate a faster response to changes in incident CO challenges. It is further desirable for gold-based catalysts to provide longer lasting protection against CO. It is also desirable to use such catalysts in respiratory protection systems that provide protection against not only CO but other airborne contaminants as well.